1. Field of the Invention
This invention relates to the separation of an adsorbate component from a liquid mixture. More particularly, it relates to an improved process and apparatus for the bulk, liquid-phase absorptive dehydration of alkanol/water azeotropic compositions.
2. Description of the Prior Art
Large quantities of ethanol, i.e. ethyl alcohol, derived from fermentation and synthesis processes, are produced annually. Ethanol is important as a solvent or extractant in the manufacture of protective coatings, nitrocellulose, smokeless powder, cosmetics and pharmaceuticals. It is a raw material for the production of ethyl esters and chloroform. It can be oxidized to acetaldehyde to produce higher molecular-weight organic chemicals, and is an important intermediate in the production of vinegar, pharmaceuticals, dyes, detergents, polishes, photographic materials and lubricants.
Important industrial sources of ethanol from agricultural materials are fermentation products of sugars obtained from blackstrap molasses generated in sugarcane mills, starchy materials such as corn and potatoes, and sulfite waste liquor from wood pulp. Synthetic ethanol has been obtained from ethylene, which was derived from petroleum fractions and natural gas liquids. Ethylene was absorbed in sulfuric acid to yield ethyl sulfates which were hydrolyzed to crude ethyl alcohol and sulfuric acid. The alcohol was subsequently fractionated to produce 95% ethanol. One of the main commercially important synthetic processes now practiced is the direct hydration of ethylene over a phosphoric acid catalyst.
In the past, there occurred a gradual shift from fermentation-derived alcohol to synthetic materials as the principal feedstock. In the recent past, synthetic ethanol comprised a large portion of the total ethanol production. This was a consequence of economic factors such as the unavailability of molasses and relatively cheap ethylene. More recently, however, other factors, such as the shut down of ethyl sulfate process plants, sharp increases in petroleum feedstock costs, the revival of fermentation facilities and the availability of sugarcane-fermentation alcohol, have all acted to change the supply balance between the two industrial sources. This brought about an increase in the annual output of fermentation-based ethanol, including potable alcohol. This increase was attributed mainly to an increasing use of "gasohol," blend of 90% gasoline and 10% ethanol. For the same time period, synthetic ethanol production also expanded. Due in part to the increasing fuel shortage, national objectives have been made for increasing the annual production of fuel ethanol.
Fermentation-based ethanol as condensed from the distillation column is typically 95-95.6 weight-% alcohol, with the balance being mainly water. This mixture is described as "azeotropic," in that vapors boiling from the liquid have the same composition as the liquid. Therefore, it is not possible to achieve a higher concentration by ordinary distillation. In order to obtain anhydrous (99.9+%) ethanol, various extraction/distillation techniques have been tried in order to "break" this azeotrope. The most successful of these processes involves the use of benzene as a third component. This, however, is a costly and energy-intensive method and also involves a material of known toxicity. Chemical means of dehydrating the azeotrope are known, such as the use of calcium oxide, but this is not practical or economical on an industrial scale because of subsequent separation problems.
Adsorptive materials, such as molecular sieves, may be effectively used in processes for the adsorptive dehydration of an azeotropic mixture of ethanol and water as an alternative to distillation or other separation method. Molecular sieve adsorbents effect separations of liquid mixtures by virtue of an adsorptive preference for one or more of the mixture components. The preference can be based on molecular size, i.e. the ability of the preferred adsorbate to enter a pore system of the molecular sieve, to the exclusion of other molecular species. In such cases the preference is absolute. Preference can also be based on the polar character of the potential adsorbates or on their relative volatility. In general the more polar and the less volatile species are preferably, i.e. selectively, adsorbed. This latter condition is that which primarily occurs in adsorptive dehydration. The commercial application of molecular sieves to liquid drying is usually conducted in conventional multi-vessel equipment. Each vessel is operated alternately in dehydration, i.e. adsorption, and regeneration, i.e. desorption, stages. A typical application involves the following series of steps:
1. Feeding the azeotrope to be dehydrated to the vessel containing a layer or zone of adsorptive material, either in an upflow or downflow direction, for a predetermined time. Usually this time will be slightly less than the time required for breakthrough of the water into the effluent.
2. Draining the vessel of the bulk of the azeotrope contained in the void spaces within the zone of adsorbent material
3. Using a countercurrent, hot regeneration fluid to remove both any residual, void-azeotrope and the adsorbed water. Both of these will normally be collected by condensation and separated out from the regeneration fluid. The regeneration fluid is usually a relatively dry, non-adsorbable gas.
4. Returning the vessel to a temperature for carrying out adsorption by subsequently passing a cool regeneration fluid through the vessel.
5. Repeating the steps 1-4.
Another dehydration application, described in U.S. Pat. No. 3,080,433 (Hengstebeck), discloses a system for dehydrating olefin feedstock. In this procedure, however, the regeneration fluid is passed through the adsorption vessel in the same direction that the feedstock passes during dehydration. Other procedures used in adsorptive dehydration disclose the use of multiple layers of adsorptive materials within the vessel, such as in U.S. Pat. No. 3,161,488 (Eastwood et al).
It is also known in the art of liquid-phase separation using solid adsorbents contained in a vertical column, to have a purified effluent from an adsorption stage withdrawn at a point intermediate in the vessel. One such separation using molecular sieves as the adsorbent material is disclosed in U.S. Pat. No. 2,985,589 (Broughton et al). The procedure utilizes a simulated moving bed effected by means of a fluid-directing device referred to as "rotary valve." Withdrawal of the product is accomplished at selected (but periodically varying) locations in a column comprising a series of interconnected layers or sorption zones. There is also provided a timely, interacting flow of regeneration fluid into the column using the same valve. Other dehydration applications using vessels wherein fluids are introduced or withdrawn through intermediate points in the vessel, include those disclosed in U.S. Pat. No. 1,541,921 (Caps), U.S. Pat. No. 2,891,007 (Caskey et al.), U.S. Pat. No. 3,382,169 (Thompson), U.S. Pat. No. 3,517,817 (Hitzel) and U.S. Pat. No. 3,617,558 (Jones). The commercial, molecular sieve liquid-drying applications disclosed in the prior art, such as for propane and butane, may use either upflow or downflow operation in the adsorption stage.
The bulk, liquid-phase dehydration of an azeotropic mixture of ethanol and water using molecular sieve adsorption requires an extremely large quantity of regeneration fluid, due to the large molecular sieve adsorbent requirements for the large amount of water to be removed. To minimize the overall regeneration fluid requirements, a closed-loop regeneration cycle is required. The resulting process cycle consists of a liquid-phase adsorption stage in the upflow direction, followed by a draining step in the downflow direction, followed by a closed-loop regeneration stage. The latter stage may comprise initial heating steps and a cooling step, both in the downflow direction.
Problems have developed in this liquid-phase dehydration procedure. First, due to the large amount of adsorbate, i.e. water, to be removed, the treating rate is extremely slow. Since the mass-transfer rate is relatively good, the use of a conventional vessel design having effluent draw-off from the top of the vessel would require the mixture to "push" the effluent out of the vessel in a plug-flow manner. However, during the adsorption stage a certain amount of effluent is retained in the zone of adsorbent material due to the retention of effluent in macropores and voids in the adsorbent zones. Since an additional amount of mixture is required to "push" the effluent from the vessel, and to overcome the increased pressure due to the plugging effect at the top of vessel, there is a loss in the amount of effluent produced during each cycle of the adsorption stage due to a corresponding increase in the amount of mixture retained in the adsorbent. Second, since the adsorbent zone is being regenerated in a flow direction countercurrent to adsorption, any adsorbate or other adsorbable components contained in the cool regeneration fluid would be deposited in the adsorbent material adjacent the effluent end of the zone or bed during adsorption. If this adsorbate level is too high, enough adsorbate could be stripped by the product effluent during the next adsorption step to exceed the purity specifications for the effluent product. In many of the prior art liquid-phase dehydration applications the presence of such water in the product is nominal. However, in the requirements for making 99+% ethanol, for such applications as the preparation of gasohol, the presence of such water in the cool regeneration fluid is unacceptable. One solution for this problem would be to reduce the capacity of the cool regeneration fluid to retain adsorbate by lowering the temperature of the fluid to less than around 15.degree. C. This, however, would require refrigeration of the regeneration fluid to ensure a low residual adsorbate level in the effluent. There is a need in the art, therefore, for improvements in the bulk, liquid-phase dehydration of such azeotropic mixtures.
It is an object of the invention to provide an improved process and apparatus for the liquid-phase adsorption of an adsorbate from a liquid mixture.
It is another object of the invention to provide an improved process and apparatus for the liquid-phase dehydration of a liquid mixture without effluent loss due to the retention effect during withdrawal of the effluent.
It is a further object of the invention to provide an improved process and apparatus for the liquid-phase dehydration of an alcohol/water azeotrope to produce a high purity effluent.
With these and other objects in mind, the invention is described in detail, with the novel features being particularly pointed out in the appended claims.